Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid-oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two O−2 ions catalytically by the cathode. The oxygen ions diffuse through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through the load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived by “reforming” hydrocarbons such as gasoline in the presence of limited oxygen, the “reformate” gas includes CO which is converted to CO2 at the anode. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to an afterburner for unspent fuel exiting the stack; and providing cooling air to the afterburner and the stack.
An enclosure for an SOFC system has two basic functions. The first function is to provide thermal insulation for some of the components which must function at an elevated temperature (700° C.–900° C.) to maintain them at that temperature for efficient operation, to protect lower temperature components outside the thermal enclosure, and to reduce the exterior temperature over the overall unit to a human-safe level. The second function is to provide structural support for mounting of individual components, mounting the system to another structure such as a vehicle, protection of the internal components from the exterior environment, and protection of the surrounding environment from the high temperatures of the fuel cell assembly.
In a solid-oxide fuel cell system, the “hot” components, e.g., the fuel cell stacks, the fuel reformer, tail gas combuster, heat exchangers, and fuel/air manifold, are contained in a “hot zone” within the thermal enclosure. The thermal enclosure is intended specifically for minimizing heat transfer to its exterior and has no significant structural or protective function for its contents. A separate and larger structural enclosure surrounds the thermal enclosure, defining a “cool zone” outside the thermal enclosure for incorporation of “cool” components, e.g., the air supply system and the electronic control system. The structural enclosure components are known in the art as a “plant support module” (PSM).
It is important that elements of the PSM be actively cooled during operation of the SOFC system. In the prior art, the elements are discrete, and such cooling is accomplished typically by employing various independent auxiliary fans or blowers, which are inefficient and which require additional power, package space, and independent ducting. Further, the heat removed from the PSM components is wasted by being discharged to the atmosphere via exhaust ports in the structural enclosure.
What is needed is a compact, integrated self-cooling PSM system for an SOFC, the system to include, at least, filtering incoming air, cooling an electronic control module and a main blower motor, and providing the resulting heated air forward as process air to the fuel cell assembly.
It is a principal object of the present invention to reduce the size and complexity of a fuel cell PSM cooling system.
It is a further object of the invention to pre-heat fuel cell process air by passing it over PSM components in need of active cooling.